Studying Living Organisms

Fish Parasite Survey

Elliott Dabill
Woodrow Wilson Biology Institute

Target age or
ability group:
Advanced biology class, may be modified for beginning biology.
Class time
Two 50-minute class periods for lab, two more for analysis.
Materials and equipment: Microscopes or magnifying lenses
Dissecting kits: tray, scalpel, forceps, probe, needle, scissors
Metric rulers
Sample jars for parasites (with labels for source)
References: Wall chart for naming fish works well, but texts are useful for names as well as fish anatomy

Summary of activity: This lab involves the survey and dissection of as many fish as possible (I used 150). Students count nematodes, cestodes and crustaceans on the fish, fill out autopsy reports, and transfer data to a chalkboard data table. The results of the entire class are graphed and used to explore coevolution.

Extensions from this activity may include electrophoresis studies of nematode proteins, Hardy-Weinberg calculations and other ecological studies. Literature searches are a natural follow-up to student questions about the huge quantity and variety of parasites found.

Prior knowledge, concepts or vocabulary necessary to complete activity:
Introduction to invertebrate parasites and life-styles. The advanced version may require introduction to population dynamics.

Teacher Instructions

Advance planning is necessary for obtaining the animals. I obtain mine from a fish packing plant that fillets the fish and makes the carcasses available for study. Thirty-gallon trash cans, plenty of ice, and a utility trailer are valuable. Disposal of carcasses may be critical as well, since your custodians may resent this kind of mess. My school has a composting pile, and some of the fish were rototilled directly into the garden.


1.To investigate the ecology of parasitism in marine teleosts.

2.To quantify variation in host infestations.

3.To practice dissection skills early in the year and learn some fish anatomy.

Questions and further study:

Data analysis usually invites more questions than answers. Virtually all of our fish were infested, but a small number had no nematodes or cestodes establish colonies. If questions are solicited from the students they can usually be sorted into parasite and host-related inquiry opportunities. Population dynamics, speciation and selection pressures are interesting follow-up lessons.

Fish Parasite Survey

How common are parasites like nematodes and cestodes (tapeworms) in fish? This lab will provide you with some background on the numbers and variety of these organisms throughout fish populations. Your textbook or other source will be useful in identifying internal and external anatomy of a fish for the initial survey, and more detailed internal anatomy of the digestive system for the worm count.

CAUTION: For these labs, wear gloves, tie hair back, and choose your clothes carefully!


A.WHOLE FISH SURVEY (do this on one fish only). On a separate sheet of paper, list the type of fish, parasites found, and the part of the fish body it came from.
1.Follow your teacher's instructions for getting equipment and your first fish.
2.Scrape along the scales of the fish and prepare a microscope slide to look for parasites of the skin. Under the pectoral fin is a good place to look.
3.Remove the operculum with scissors to carefully study the gills for attached parasites.
4.Study the inside of the mouth for visible parasites; scrape the roof of the mouth for a smear.

B.COELOM SURVEY: Do this on as many fish as you can for a large class sample size. Results from each fish are to be recorded on an autopsy report table, and, at the end of the lab, put on the chalkboard data table for the entire class to add to. Label a sample bottle with the type of fish, date, and type of parasites; save the parasites for later study.
1.Open the coelom by making a ventral surface cut from the anus forward to an imaginary line at the posterior portion of the operculum. Now cut out the entire side of the coelom by cutting a rectangle of skin from behind the operculum, anterior to the anus, and ventral to the backbone.
2. Follow the digestive system from the esophagus to the anus and list the number of nematodes found on each major organ (they usually resemble a coiled spring). List the nematodes you find on the other organs, too, like the liver, the wall of the coelom (the peritoneum), etc.
3. Cut out the small and large intestines and use a wash bottle to flush out the inside so that tapeworms will come out the back end.
4. Dispose of the fish remains where directed and get another fish if there is enough time before clean-up. You will need at least 10 minutes for washing equipment and your lab area.


A.Autopsy Results

Fish Number: Fish Type: Length:
Parasites in... Liver: Stomach:
Small Intestine: Large Intestine:
Other: Other:

B. Make a bar graph of the class data. Plot the number of parasites per fish organ for both schooling fish and bottom fish.


1.From your graph, describe the frequency of parasites in the marine fish we studied.

2.If the fish and parasite genotypes were stable and balanced with each other, describe the changes that might occur if a single mutation left a fish more able to resist the parasites.

3.What if the fish mutation in question #2 was followed by increased numbers of nematode variants that could invade the new fish, but made them less adapted to the original fish?

4.Describe how the interactions between parasites and hosts might lead to new species.


Noble, E.R. and Noble, G. A. Parasitology: The Biology of Animal Parasites. London: Lea and Febiger Publishing, 1971.

Rollinson, D. and Anderson, R. M. (editors). The Ecology and Genetics of Host-Parasite Interactions. Orlando, FL: Academic Press, 1985.

Smith, J. D. Introduction to Animal Parasitology. Cambridge University Press, 1994.

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